The present invention relates to a novel seal facing for use with a mechanical rotary seal to prevent dry running during the pumping of volatile liquids.
A number of structural modifications to seal facings have been suggested for preventing the dry running condition encountered in gas seals. For example, Sedy U.S. Pat. No. 4,212,475 and Pecht et al. U.S. Pat. No. 5,217,233 are examples of grooved structures in modified seal facings which allegedly prevent the dry running condition in gas seals.
These modified seal facings in gas seals have been applied to fluid pumps. However, in such instances, the seal facing of one of the mechanical seals includes spiral shaped grooves or channels which are so positioned that when the mechanical seal is rotated, fluid is pumped into the edge of one of the sealing rings to form a barrier therebetween to prevent contact between the mechanical face seals under such dynamic conditions. Accordingly, such seal facings have notched faces with bottoms of the grooves parallel to the face. See, for example, Amundson et al. U.S. Pat. No. 4,889,348 and Kimura et al. U.S. Pat. No. 5,180,173. Also, Sedy U.S. Pat. No. 5,531,458 discloses progressively narrower grooves to somehow provide a pressure buildup. However, because the mating face seal surfaces must be lapped to provide planar surfaces during dynamic operation to prevent leakage of liquified gases, these flat facing surfaces tend to ring together. This is particularly manifested in the pumping of volatile liquids, such as ammonia, which results in the build-up of heat and the destruction of the face seal. Thus, the prior art suggested modifications to the mating surfaces have not prevented a dry running condition when utilized to pump volatile liquids.
The Environmental Protection Agency has issued stringent standards for pumps which are used to pump certain hazardous fluids. For example, the standards for light liquid service require dual mechanical seal systems and a barrier fluid system with each dual mechanical seal system. Also, the barrier fluid system must be operated with the barrier fluid at a pressure that is at all times greater than the pump stuffing box pressure or equipped with a barrier fluid degassing reservoir that is connected by a closed-vent system to a control device. An alternative requirement is that the pump be designed with no externally actuated shaft penetrating the impeller casing.